We turn to Russia striking back at the U.S. This morning with president Putin retaliating for sanctions over the crisis in Ukraine by banning American food imports including meat, fish, fruits and vegetables. ABC's Jon Karl is at the white house and this comes as the administration is voicing new concern about the Russian buildup on the border of Ukraine. Reporter: In fact, George, chuck Hagel, the defense secretary, is warning there is an increasing threat of a Russian invasion of Ukraine. U.s. Officials say there are 20,000 Russian troops now amassed on the border and that they are armed with the kind of heavy weaponry that they would need to conduct a full-scale invasion of eastern Ukraine and on the sanctions front, Russia is not only banning food imports from Russian and Europe but they are threatening to ban western airlines from flying over Russian territory. Something that could dramatically increase costs and flight times for flights to and from Asia so the bottom line, George, is that U.S. Sanctions on Russia appear to be doing zero to convince Vladimir Putin to back down. And the president was saying at his press conference yesterday which you were at he wasn't prepared yet to offer more military aid to Ukraine but was looking at it on a day-to-day basis. Another announcement from Russia they'll give and a poke in the eye as well giving more time to Edward snowdens the fugitive NSA leaker. He has three mores of asylum in Russia. A reminder russia/u.s. Tensions go beyond Ukraine. Snowden, of course, is the most high-profile, most wanted American international fugitive there is and now he'll have at least another three years of safe haven in Russia. Jon, thanks very much.

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